Temperature Measurement with Thermistors...temperature and resistance is T= k R A thermistor with...

Temperature Measurement with Thermistors

Gerald Recktenwald

Portland State University

Department of Mechanical Engineering

[email protected]

March 3, 2019

ME 121: Introduction to Systems and Control

Temperature Measurement

Temperature can be measured with many devices

• Liquid bulb thermometers

• Gas bulb thermometers

• bimetal indicators

• RTD: resistance temperature detectors (Platinum wire)

• thermocouples

• thermistors

• IC sensors

• Optical sensors

. Pyrometers

. Infrared detectors/cameras

. liquid crystals

ME 121: Introduction to Systems and Control page 1

Thermocouples: Overview

• Principle of operation

• Wire types: B, E, J, K, N, R, S, T

• Formats: prefab, homemade, fast response, slow response

• Circuit diagrams: reference junction compensation

• Good practice


Seebeck Effect (1)

Temperature gradient in a conductor induces a voltage potential

T1

T2

Voltmeter

E12

E12 = σ(T2 − T1) (1)

where σ is the average Seebeck coefficient for the range T1 ≤ T ≤ T2.


EMF Relationships for Thermocouples (2)

Nominal values of Seebeck Coefficient

Type

Metal

+ −Seebeck

Coefficient

Temperature

Range

J Iron Constantan 50µV/C −210 to +760 C

K Nickel-

Chromium

Nickel 39µV/C −270 to +1372 C

T Copper Constantan 38µV/C −270 to +400 C

σ values are small, so the voltage output from thermocouples is small, typically on the

order of 10−3 V.


Thermocouple Amplifier

Although it’s possible, and in come cases preferable, to measure thermocouple signals

with a laboratory-grade DMM, for our purposes in ME 120, 121, 122, we can use a simple

thermocouple amplifier chip.


IC Temperature Sensors (1)

• Semiconductor-based temperature sensors for thermocouple reference-junction

compensation

• Packaged suitable for inclusion in a circuit board

• Variety of outputs: analog (voltage or current) and digital

• More useful for a manufactured product or as part of a control system than as

laboratory instrumentation.

Examples (circa 2010)

Manufacturer Part number

Analog Devices AD590, AD22103, TMP35, TMP36, TMP37

Dallas Semiconductor DS1621, DS18B20

Maxim Max31885, Max675, REF-01, LM45

National Instruments LM35, LM335, LM75, LM78


IC Temperature Sensors (2)

Example: TMP36 from Analog Devices

Don’t confuse the TO-92-3 package with a transistor!

Low Voltage Temperature Sensors TMP35/TMP36/TMP37

Rev. E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©1996–2008 Analog Devices, Inc. All rights reserved.

FEATURES Low voltage operation (2.7 V to 5.5 V) Calibrated directly in °C 10 mV/°C scale factor (20 mV/°C on TMP37) ±2°C accuracy over temperature (typ) ±0.5°C linearity (typ) Stable with large capacitive loads Specified !40°C to +125°C, operation to +150°C Less than 50 µA quiescent current Shutdown current 0.5 µA max Low self-heating

APPLICATIONS Environmental control systems Thermal protection Industrial process control Fire alarms Power system monitors CPU thermal management

GENERAL DESCRIPTION The TMP35/TMP36/TMP37 are low voltage, precision centi-grade temperature sensors. They provide a voltage output that is linearly proportional to the Celsius (centigrade) temperature. The TMP35/ TMP36/TMP37 do not require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the !40°C to +125°C temperature range.

The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and ADCs. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maxi-mum. The supply current runs well below 50 µA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut the supply current to less than 0.5 µA.

FUNCTIONAL BLOCK DIAGRAM !"#$%&'("$)*$+'+",

"*-)#.-)/*01

)234+5)23465)234(

00337-001

Figure 1.

PIN CONFIGURATIONS

7

&

4

+

8

)*3$"9:0%1;<$<;$#=>?@,

1A$B$1*$A*11:A)

"*-)

#.-)/*01

C1/

1A

!"#

00337-002

Figure 2. RJ-5 (SOT-23)

7

&

4

8

D

(

6

+

)*3$"9:0%1;<$<;$#=>?@,

1A$B$1*$A*11:A)

"*-)

#.-)/*01

1A

1A

!"#

1A

1A

C1/

00337-003

Figure 3. R-8 (SOIC_N)

7 4&

E*))*2$"9:0%1;<$<;$#=>?@,

391$7F$!"#G$391$&F$"*-)G$391$4F$C1/ 00337-004

Figure 4. T-3 (TO-92)

The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from !40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C.

The TMP37 is intended for applications over the range of 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply.

The TMP35/TMP36/TMP37 are available in low cost 3-lead TO-92, 8-lead SOIC_N, and 5-lead SOT-23 surface-mount packages.

Low Voltage Temperature Sensors TMP35/TMP36/TMP37

Rev. E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©1996–2008 Analog Devices, Inc. All rights reserved.

FEATURES Low voltage operation (2.7 V to 5.5 V) Calibrated directly in °C 10 mV/°C scale factor (20 mV/°C on TMP37) ±2°C accuracy over temperature (typ) ±0.5°C linearity (typ) Stable with large capacitive loads Specified !40°C to +125°C, operation to +150°C Less than 50 µA quiescent current Shutdown current 0.5 µA max Low self-heating

APPLICATIONS Environmental control systems Thermal protection Industrial process control Fire alarms Power system monitors CPU thermal management

GENERAL DESCRIPTION The TMP35/TMP36/TMP37 are low voltage, precision centi-grade temperature sensors. They provide a voltage output that is linearly proportional to the Celsius (centigrade) temperature. The TMP35/ TMP36/TMP37 do not require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the !40°C to +125°C temperature range.

The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and ADCs. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maxi-mum. The supply current runs well below 50 µA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut the supply current to less than 0.5 µA.

FUNCTIONAL BLOCK DIAGRAM !"#$%&'("$)*$+'+",

"*-)#.-)/*01

)234+5)23465)234(

00337-001

Figure 1.

PIN CONFIGURATIONS

7

&

4

+

8

)*3$"9:0%1;<$<;$#=>?@,

1A$B$1*$A*11:A)

"*-)

#.-)/*01

C1/

1A

!"#

00337-002

Figure 2. RJ-5 (SOT-23)

7

&

4

8

D

(

6

+

)*3$"9:0%1;<$<;$#=>?@,

1A$B$1*$A*11:A)

"*-)

#.-)/*01

1A

1A

!"#

1A

1A

C1/

00337-003

Figure 3. R-8 (SOIC_N)

7 4&

E*))*2$"9:0%1;<$<;$#=>?@,

391$7F$!"#G$391$&F$"*-)G$391$4F$C1/ 00337-004

Figure 4. T-3 (TO-92)

The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from !40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C.

The TMP37 is intended for applications over the range of 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply.

The TMP35/TMP36/TMP37 are available in low cost 3-lead TO-92, 8-lead SOIC_N, and 5-lead SOT-23 surface-mount packages.

See, e.g., part number TMP36GT9Z-NDfrom www.digikey.com.$1.42 each (Qty 1) in Feb 2013

See http://learn.adafruit.com/

tmp36-temperature-sensor/

overview for instructions on how to usethe TMP36.


Thermistors (1)

A thermistor is an electrical resistor used to

measure temperature. A thermistor is designed

such that its resistance varies with temperature in

a repeatable way.

A simple model for the relationship between

temperature and resistance is

∆T = k∆R

A thermistor with k > 0 is said to have a positive

temperature coefficient (PTC). A thermistor with

k < 0 is said to have a negative temperature

coefficient (NTC).

Photo from YSI web site:www.ysitemperature.com

The T = F (R) relationship for thermistors is nonlinear. The temperature coefficient k

is the slope in the curve over a narrow range of temperatures and resistances.


Thermistors (2)

• NTC thermistors are semiconductor materials with a well-defined variation electrical

resistance with temperature

• Mass-produced thermistors are interchangeable: to within a tolerance the thermistors

obey the same T = F (R) relationship.

• Measure resistance, e.g., with a multimeter

• Convert resistance to temperature with calibration equation

Note: The Arduino cannot measure resistance. We will use a voltage divider to

indicate the change in resistance as a change in voltage.


Thermistors (3)

Advantages

• Output is directly related to absolute temperature – no reference junction needed.

• Relatively easy to measure resistance or convert resistance to voltage with a voltage

divider.

• High precision thermistors have interchangeable tolerances of ±0.5 C.

Disadvantages

• Possible self-heating error

. Each measurement applies current to resistor from precision current source

. Measure voltage drop ∆V , then compute resistance from known current and ∆V .

. Repeated measurements in rapid succession can cause thermistor to heat up

• Precision thermistors are more expensive than thermocouples for comparable accuracy:

$10 to $20/each versus $1/each per junction. Thermistors costing less than $1 each

are available from electronic component sellers, e.g. Digikey or Newark.

• More difficult to apply for rapid transients due to slow(er) response and self-heating


Thermistors (4)

Calibration uses the Steinhart-Hart equation

T =1

c1 + c2 lnR + c3(lnR)3

Nominal resistance is controllable by

manufacturing.

Typical resistances at 21 C:

10 kΩ, 20 kΩ, . . . 100 kΩ. 5 10 15 20 25 300

5

10

15

20

25

30

35

40

45

50

Resistance (kΩ)

T (°

C)

Data

Curve Fit


Resistance Measurement

Resistance can be measured if a precision current

source is available.

If I is known and V is measured, then R is obtained

with Ohm’s law

R =V

I

R V

I

For a typical ohmmeter, the current source and voltage

measurement are inside the device. The leads connect

the internal current source to the external resistance

element being measured.

RVI

leadsohmmeter


Direct Resistance Measurement of Thermistors (1)

Two-wire resistance measurement: RT =V

I.

Ohmmeter

Thermistor

RTV

Resistance in the lead wires can contribute to inaccuracy in the temperature measurement.


Direct Resistance Measurement of Thermistors (2)

Four-wire resistance measurement eliminates the lead resistance1

Ohmmeter

Rlead

ThermistorRleadRT

Rlead

Rlead

V

1Sketch adapted from Hints for Making Better Digital Multimeter Measurements, Agilent Technologies Corporation,www.agilent.com.


A Voltage Divider for Thermistors (1)

Using an Arduino, we do not have ready access to a

precision voltage source. We could assemble a board

using high precision voltage sources, but for less effort

we could just buy a temperature measurement chip

like the LM334 or TMP36.

Instead, we will use our familiar strategy of measuring

resistance with a voltage divider.

thermistor

10 kΩ

5V

Analog input


Arduino code for Thermistor measurement

int thermistor_reading( int power_pin, int read_pin)

int reading;

digitalWrite(power_pin, HIGH);

delay(100);

reading = analogRead(read_pin);

digitalWrite(power_pin, LOW);

return(reading);

float thermistor_reading_ave( int power_pin, int read_pin, int nave)

int i, reading;

float sum;

digitalWrite(power_pin, HIGH);

delay(10);

for (i=1; i<=nave; i++)

sum += analogRead(read_pin);

digitalWrite(power_pin, LOW);

return(sum/float(nave));


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